Hindlimb unloading and female gender attenuate baroreflex-mediated sympathoexcitation

By: C. Michael Foley and Patrick J. Mueller

American Journal of Physiology - Regulatory, Integrative and Comparative Physiology, July 2005

Being exposed to periods of microgravity or bed rest can result in physiological adaptations, such as an increased susceptibility to orthostatic intolerance. Being returned to a normal 1 G environment or an upright position after exposure to microgravity or bed rest seems to cause more orthostatic intolerance in females compared to males. One cause of orthostatic intolerance may be decreased arterial baroreflex compensation. The purpose of this experiment was to test the hypothesis that female rats would exhibit a greater reduction in arterial baroreflex function after hindlimb unloading (HU) compared to female rats. 

To test this hypothesis, male and female rats were randomly assigned to the HU group or the cage control (CC) group. Female rats were examined, and vaginal swabs were taken daily to track the stages of their estrous cycles. All rats underwent surgery to implant the suspension apparatus through the 5th and 7th sacral intervertebral space. The animals were allowed to recover and went though an acclimation period before being subject to 13-15 days of suspension (HU group) or cage time (CC group). After 13-15 days of these conditions the HU rats were taken down from the suspension apparatus, and all rats were placed in a horizontal position. Then, arterial blood pressure, mean arterial pressure (MAP), heart rate (HR) and nerve activity were recorded for each of the rats. For the nerve activity, the absolute renal sympathetic nerve activity (RSNA) was determined by the recorded nerve activity present after the background noise was removed. After the experiment was over, all rats were euthanized so their soleus and plantaris muscles and adrenal glands could be removed to be weighed and checked for atrophy. 

There were many results produced from this study. First, there was evidence of atrophy of the soleus and plantaris muscles in the male and female HU groups compared to the male and female CC groups. Second, resting MAP was lower in the females compared to the males, and higher in the HU groups compared to the CC groups. Third, resting HR was higher in the females compared to the males, but the baroreflex control of HR was similar among all groups. Fourth, both gender and HU affected the baroreflex control of the RSNA. The female HU group had the least baroreflex sympathetic ability to increase RSNA in response to returning to the horizontal position. Overall, the baroreflex control of HR was similar in males and females, but there was an upward shift in the response in the HU groups of both genders. This produces the major finding that: both female gender and HU result in blunted baroreflex-mediated sympathoexcitation. This is consistent with other studies involving humans that found that females are more affected by orthostatic intolerance after return from microgravity or bed rest conditions.